Question

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Which of the following is necessary for a process to be spontaneous? A) Delta H_sys < 0 B) Delta S_sys > 0 C) Delta S < 0 D) Delta S_univ > 0 E) Delta G_sys = 0 Which reaction has the largest Delta S degree? A) 2 N_2H_4(g) rightarrow 2 NH_3(g) + H_2(g) B) O_2(g) + 2 H_2(g) rightarrow 2 H_2O(l) C) O_2(g) + 2 H_2(g) rightarrow 2H_2O(g) D) N_2(g) + 3H_2(g) rightarrow 2NH_3(g) E) 2NO(g) rightarrow N_2O_2(g) Which of the following is true for a system at equilibrium? A) Delta S degree_sys = Delta S degree_surr B) Delta S degree_sys = -Delta S degree_surr C) Delta S degree_sys = Delta S degree_surr = 0 D) Delta S degree_univ > 0 E) None of these is a sufficient condition. The third law of thermodynamics is: A) The entropy of the universe is increasing. B) The entropy of the universe is constant. C) The entropy at T = 0K is zero. D) The energy of the universe is constant. E) The entropy of all elements are zero.

Answer 1

29. For a reaction to be spontaneous, thee are two conditions:

1. Tendency to achieve minimum gibbs free energy i.e. $\Delta G$ <0

2. Tendency to achieve maximum randomness i.e. $\Delta S_{system}$ >0

$\Delta S_{system}$ >0 is necessary for spontaneous process

30. For a reaction involving only gases, a decrease in the number of moles on the product side means lower entropy. An increase in the number of moles on the product side means higher entropy.

For a reaction involving multiple phases, the production of a gas typically increases the entropy much more than any increase in moles of a liquid.

Out of all the reactions, 2N₂H₄ -----> 2NH₃ + H₂ is a reaction where number of moles are increasing on the product side whereas all the reactions have lesser number of moles on product side and more number of moles in reactant side. So as mentioned in statement above, this reaction proceeds with higher entropy change

31. For a system at equilibrium, the change of entropy of system = - change of entropy of surroundings so that total change in entropy = 0

Therefore, $\Delta S_{system} = - \Delta S_{surroundings}$

32. The third law of thermodynamics states the entropy of a perfect crystal is zero when the temperature of the crystal is equal to absolute zero (0 K).

Thus, entropy at T= 0K is zero